Pavement Chassis Setup - Asphalt Race Car Preparation

As each season comes along, our knowledge of what we need and desire grows. At some point, we take in enough information to allow us to avoid the mistakes of the past and get ahead of the curve in setting up our asphalt cars. Trends are a part of this learning curve and greatly influence some, but not all racers.

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A trend must prove itself in order to stand the test of time. It must show a consistent gain in performance and a logical ideology to the bulk of race teams. And, it must be economically applied. We are at a crossroads at this point in time with setup technology in asphalt racing.

The use of softer front springs, larger sway bars, and big spring split in the rear has swept across the country for more than five years now with mixed results. I talk to lots of racers representing every form of circle track racing in every region of the country and beyond. What I hear I feel is representative, although those who dive head first into the BBSS mix not because of a perceived and measured expectation of success but because of the "see monkey, do monkey" attitude, will never share what they are doing.

The message I get from those who have worked this trend for a sufficient number of years is one of the following two types: 1) a continuing and constant search for balance in the car and a loss of success on the track, or 2) a returning to the more conventional setups because they have experienced number 1 above. I don't get too many reports of continual, repeated wins and successes. That troubles me.

3/12What has evolved is a design for the dynamic Moment Center that is more to the left of centerline. This provides a longer moment arm and makes the front more efficient. Efficiency helps retain loading on the left front tire to help the car turn.

And as for you Modified and Stocker class teams, don't even think about it. Yes, I get inquiries about how to apply the BBSS setups to both of these types of cars. We have been softening those setups over the years, but never intended to go to coil bind or bump rubbers. It just won't work. So don't try it.

So, again this year as in past years, we will present what is admittedly the formula for success related more for the conventional and what we call soft conventional setups mainly because these are the ones that produce more consistency and do win races.

I have always believed in priorities because you can make gains faster when you solve the high priorities problems first. Smaller gains can come later on after the more important aspects of setup are resolved. And, some aspects of chassis setup build on other aspects. So, here is, in order of logic and importance, a list of setup parameters we need to address to make our cars fast and consistent.

The very first step in the process of preparing for the new season is to consider all performance-related items and how they worked last season. Plan out changes that could help improve performance or durability. Both of these are necessary components that will be needed to win championships. Here are what we consider to be the 10 most important areas of chassis setup with No. 1 being the most important.

4/12We want minimal rear steer as the car negotiates the turns. A small amount of steer to the left will help tighten the car. If overdone, the car will push. For three-link cars, set the control arm angle so that the front mount is 1/3 of the total travel distance higher than the rear mount. For cars running stiff right rear spring, remember that the travel will be much less due to the stiffness. You must change your trailing arm angles when changing to a stiffer RR spring.

1. Front End Geometry
We always start with the front end geometry on any race car. The settings, including the moment center location, really do dictate how all of the other parts and pieces of setup will work. If this component on your car is not right, then the whole car will suffer, no matter what else you do. Numerous car builders have come to realize the truth in the above statements.

A tight condition is the number one complaint from drivers. The number one reason a car will be tight and not want to turn is because the front end is not designed properly. The moment center must be located correctly for your type of racing and the cambers must be set, again toward your setup style and track conditions.

The dynamics of the moment center and the effects of camber change have been explained before. We have continually pressed these issues because of the extreme importance they have. Long gone are the days of saying that the MC is not important.

The dynamics of the moment center and the effects of camber change have been explained before. We have continually pressed these issues because of the extreme importance they have. Long gone are the days of saying that the MC is not important.

5/12Alignment is a critical part of your setup package. This includes toe, bumpsteer, Ackermann, and tire contact patch relationships. A few moments spent checking and refining alignment will make a significant difference in how your car performs. There are racing companies who have built their whole product line around these aspects.

The influence of the location of the front MC can be compared to a sliding scale. If you could slide the MC to the right, or outside of the turn for you road racers, the front end will get stiffer. Sliding the MC left and to the inside of the turn makes the suspension softer. The effect is huge. It is this sliding scale situation that determines the stiffness of your front end.

Cars that don't turn well are very likely to have poor MC designs. I can't tell you how many times I have refined the MC location in a car and had it totally change the way the car turned, for the better. I have had a lot of feedback from teams who did the same with the same results.

2. Rear Geometry
The second most important item in the setup arsenal is the rear geometry layout in your car. The components that locate the rear end must be evaluated and set correctly. The control arm angles affect the rear steer and the third-link angle can redistribute load upon acceleration. On a metric four-link car, the four control arms determine the rear moment center height too.

It is not advantageous to have the rearend steer to the right at any time on asphalt. A slight amount of rear steer to the left has been shown to help provide more traction at the rear and bite off the corners where it is needed. But the most useful rear steer will only occur on acceleration and not at mid-turn.

6/12It's important to know the rates for your springs and shocks. Always rate the springs in the range of compressed height that they will operate in. Dyno your shocks or have someone else do it. Now would be a good time to rebuild them with new seals, and so on.

On a three-link rear suspension you should have the front of your right-side control arm higher than the rear mount by 1/3 of the total amount it will travel in the turns. With setups that use a very stiff right rear spring, the angle of the right trailing arm will need to be less than when using a conventional softer spring because that corner will move less.

3. Steering Geometry
The steering system in your car must be evaluated and any negative characteristics must be eliminated. Negative aspects might include excessive bumpsteer (more than 0.030 bump in or out in for each inch of travel is considered negative by most designers), excessive Ackermann (more than a 1/4 degree added steer in either front wheel in 10 degrees of steering input is considered excessive), and incorrect steering quickness.

Eliminate most of your bumpsteer and Ackermann and install the correct steering ratio for your track that would suite the driver. Ackermann is easily checked by using a laser system or strings. If all of these issues are evaluated and corrected, then you can move on.

4. Alignment
It has been found that the misalignment of your tires/wheels presents serious drawbacks to a finely tuned chassis and setup. Alignment issues are defined as: A. rearend alignment, B. contact patch alignment, C. driveshaft to pinion/transmission alignment, and D. engine alignment.

The rearend needs to be aligned at 90 degrees to the centerline of the chassis and/or to a line through the center of the right-side tire contact patches. The right-side tire contact patches will also need to be inline with the right front tire pointed straight ahead.

The driveshaft alignment is critical from the standpoint of mechanical efficiency. Loss of efficiency can rob power from the drivetrain due to the generation of vibrations and harmonics that are also damaging to the bearings. The overall general rule is that the angles between the driveshaft and both the pinion shaft and the transmission output shaft need to be equal and in opposite directions. The less angle the better.

7/12The stiffness of the shocks for compression and rebound should match the spring stiffness you are running. Remember that springs resist compression and promote rebound, so you need more rebound control than compression control. Adjustable shocks make tuning the car much easier.

The driveshaft doesn't know which view these angles are resulting from, just that they are equal and opposite. If we have a top view difference in alignment between the transmission output shaft and the pinion shaft and they are parallel, then the angle to the driveshaft created by that misalignment might be sufficient to provide the needed angular differential needed for loading the U-joint bearings and reducing negative harmonics.

A 1 1/2-inch top view misalignment with a 44-inch driveshaft results in nearly 2 degrees of angle at both the tranny and the pinion shafts. The engine should always be aligned perpendicular to the rearend and/or parallel to the centerline of the car. This would mean we could align the driveshaft from a side view inline with the tranny shaft and the pinion shaft with no angular deflection.

5. Setup Balance
Balance is spoken of in all types of motorsports these days, even F1. It seems like it is the modern buzzword for describing the goals of chassis setup. Here is an explanation of balance related to the dynamics of the race car.

The setup we choose needs to be arranged so that the dynamics are balanced between the front and the rear suspensions. Each suspension system desires to do its own thing when lateral forces are introduced from the car going through the turns. These desires are directly influenced by the spring stiffness, location, and spring split, the sprung weight the system has to support, along with the moment center locations and other settings.

8/12If the brake bias is at or nearly out of range for the adjuster in your car, it is time to think about changing the size of your master cylinders and/or installing different compound brake pads. It is not recommended to run the adjusters very far off center because of the inconsistencies that will result. Note: the cylinder on the left is the front brake master and is a smaller diameter than the next one which is the rear brake master. This provides more front braking force which is needed.

Each end has its own moment arm length and resistance to roll as well as other factors. The bottom line is that at mid-turn, each end will want to roll to its own degree of angle. That is the best description of the result of the dynamic force that influences each system. If those desired angles are different, then we term the setup unbalanced.

Unbalanced setups exhibit easily observed characteristics, such as unusually high degree of wear and temperature on one tire versus the other tires. The car may or may not be neutral in handling, but the handling will not be consistent. You must try to determine if your setup is balanced and then if not, make the necessary changes to bring it into a balanced state.

6. Shocks
Once you have evaluated all of the above and feel fairly confident that the car is set up correctly, you should then work to tune the transitions into and off of the corners with the shocks. The overall work that a shock does is to resist the rebounding of the springs and control the speed of compression. Since the spring promotes rebound and resists compression as inherent properties, then the shock rate of compression control must be less than the rate of rebound control.

The amount of difference you need is directly influenced by the installed motion ratio of the spring and the spring rate. A very soft spring would need more compression rate and less rebound rate, whereas a stiff spring would need a lot of rebound rate and much less compression rate. These are the general rules unless you are trying to "tie down" a corner. However, tie down shocks are becoming less and less desirable.

Shocks affect the motion of the corners of the car and therefore the placement of loads during transitional periods. If one corner of the car is shocked stiff, then as that corner desires to move in compression, more load will be retained by that corner as well as the opposite diagonal corner of the car during the compression cycle only.

If the same stiffly shocked corner is in rebound, less of the overall load will be retained by that corner, and its diagonal corner as well, during the rebound cycle only. That is the essence of shock technology related to handling influences. Plan your shock layout by comparing the stiffness of one to the other corners and to the spring stiffness at the corner you are trying to control.

9/12The third link provides force onto the rear axle assembly as the car accelerates. When we install the link at an angle (front down), as the rear end tries to rotate moving the pinion up, the link tries to straighten and become parallel to the force that is applied to it. This places a force downward onto the rear end. Moving the link left and right changes the distribution of that force on the two rear tires.

7. Brake Bias
Once the setup has been balanced and the shocks are decided upon, we need to evaluate the turn-entry characteristics and the fact that brake bias is a very important influence at this segment of the track. We do not want to try to solve turn entry problems with the brake bias, we only need to make sure the car stays neutral in handling when the brakes are applied.

Brake bias influence can be easily determined by entering the corner with medium to heavy braking first and then entering with light braking to see if there is a difference. If there is, try to adjust the brake bias to eliminate the adverse condition.

Once you've made the entry to the corner balanced, check to see if the adjuster is centered. If it is too far to one side, then changes to the brake master cylinder sizes and/or the pad compounds might need to be made in order to maintain a centered-bias adjuster. Off-centered adjusters can be very inconsistent.

8. Bite Off The Corners
In situations where the exit portion of the track provides less traction and/or the corner is more flat, we might need to develop more rear traction upon acceleration. Just giving the car more rear traction, period, does not help us if the car becomes too tight in the middle of the turns and we could end up with the reverse of what we need.

10/12A trick that has been used in the past to improve rear bite when accelerating involves mounting the left spring in front of the axle and the right spring behind. When using a pull-bar, as the rear end rotates, load is added to the left rear tire and as well the right front tire and load is taken off the right rear tire and the left front tire. This increases the crossweight percent thereby tightening the car.

We must develop ways to create more rear traction on acceleration only. There are ways to do that without changing the handling at other points around the racetrack. One way is to have a rear spring split, where the right rear (RR) spring has less spring rate than the LR spring. This is very "old school" but when applied with a balanced setup, very effective. This develops more crossweight as the car squats on acceleration. Stock cars using the metric-style four-link rear suspension usually need to do this just to achieve a balanced setup with the high rear moment centers in those cars.

Another way to gain bite that we have described in the past involves the use of a spring-loaded pull-bar that allows a certain amount of rear end rotation. The idea is to steer the car using different height holes for the rear control arm mounts. As the rear end rotates on acceleration, the left wheel moves rearward more so than the right wheel, creating a slight amount of rear steer to the left. We are only talking about a difference of 0.040- to 0.060-inch, but that is enough to help stabilize our car on exit and provide added bite.

Moving the pull-bar or just the third link to the left increases the loading on the LR tire during acceleration. Be careful not to overdue that.

9. The Antis
Antidive and antisquat are mechanical influences that can help our transitional phases of entry and exit. Antidive helps prevent sudden nose dive on entry by mechanically resisting the downward motion of the suspension using the rotational forces created through braking.

As the front brakes are applied, the caliper grabs the rotor and the motion of the wheel/rotor tries to rotate the spindle. This force is resisted by the ball joints and control amrs. The upper BJ is trying to be forced in a forward direction and the lower BJ is trying to be forced in a rearward direction.

If we arrange our control arm angles, from a side view, for antidive, then as the car dives the upper BJ would move to the rear and the bottom BJ would move to the front. Since the braking forces are in the opposite direction, there is a serious resistant force created which helps prevent the front suspension from moving in compression too quickly while braking.

11/12This diagram illustrates the action and reaction of antidive forces. The braking force tries to rotate the spindle in a clockwise direction (right front viewed from the outside). As the car tries to dive, the angle of the control arms, from this view, acts to try to rotate the spindle counter-clockwise. These opposing forces cause a resistance to diving under braking. This is how antidive works.

The amount of resistance is directly related to the degree of side view angle we put in our control arms and the amount of brake force used. The left side suspension usually is designed with about half the angle of the right side in a conventional design. For the Big Bar, Soft Spring setups, teams often introduce Pro-dive into the left front suspension to encourage rapid dive on entry to get the left front down quickly. I don't really encourage that method.

Antisquat results from the third link trying to straighten out, or become more horizontal as the car accelerates and the rear end desires to rotate. The more third link angle you have, the more antisquat there is. The lateral location of the third link can affect the distribution of load among the two rear tires that results from acceleration and antisquat.

Antisquat is detrimental to corner entry. So, there is a limit to how much you can get away with and still have a decent corner entry. Roughly 8-10 degrees of third link angle is sufficient to promote antisquat and not hurt your corner entry.

12/12Aero is the very last thing you need to think about. You might not need the aero efficiency of this Cup car, but some considerations can help. Reduce drag first and then concentrate on improving downforce. The idea is to create low pressure areas under the front and rear decks. Look for past articles we have presented to find out how to do that.

10. Aero Package
The very last thing you need to worry about is your aero package. I'm not saying this is not important to some degree, but on short tracks I would stress that aero downforce is over-rated in most cases.

The reason I say that with confidence, is because I have gone up against more aero efficient cars with setups and body configurations that were aero-deficient and still out ran them. Still, teams want the most they can get out of their cars and if all of the above nine items are in order, by all means, go ahead with aero tweaking.

Try to understand how aero downforce is created and then configure your car so that you take advantage of every area where you could produce more downforce. Remember that drag is an important aspect of aero design. Do not seek aero downforce at the expense of aero drag increase.

Conclusion
No matter which stiffness you decide to go with, the most important aspect of setup is balance, and you achieve that balance with the correct combination of springs, moment centers, sway bars, and load distribution.

It may take a few test sessions to help you determine your balance, but if you observe the indicators correctly, then tuning the car for dynamic balance can be done. Then all you have to do is maintain that balance throughout the season and that means resisting making changes that take you outside that envelope. Good luck.